1. Field of the Invention
This invention relates to apparatus for detection and display of neutrons and, more particularly, to apparatus for improving the sensitivity and temporal resolution of neutron detectors.
2. Prior Art
The temporal distribution of neutrons emanating from point sources, such as an inertial confinement fusion targets used in fusion reactors, is difficult to accurately measure due to the very short time duration of the fusion reaction. Meaningful measurements for analysis of a burn history must be made on a neutron flux which has a temporal duration of about 100 picoseconds. Obviously, measurement apparatus which either is not fast enough to follow variations in the neutron flux or which introduces large temporal dispersion into the measurement data is not useful. Conventional detectors and measurement schemes are not fast enough, nor can they adequately resolve the temporal variations in the neutron flux over such a short time period.
In related areas of technology, measurements of very short-duration light and x-ray pulses of large flux magnitude have been made using very fast photodetectors. The output of these fast photodetectors feeds an x-ray streak camera, which is capable of resolving time differences on the order of 10 picoseconds or less. A typical x-ray streak camera provides a continuous exposure for a very short time interval and achieves good time resolution by rapidly changing the position, or streaking, the image on the recording surface, which may be film or a phosphor screen. The image is rapidly moved over the recording surface by using either a rapidly rotating mirror or deflection plates. X-ray streak cameras have been previously used for measurement of light or x-ray fluxes of relatively large magnitude. For x-ray measurements of large flux magnitude, the x-rays are collimated through a small slit. Because of the relatively low density of neutron fluxes provided from inertial confinement targets, slits are not practical for collimating a neutron flux.
Various types of neutron flux detectors are known in the prior art which utilize secondary emission of electrons dragged by fission fragments from materials such as uranium dioxide. These secondary electrons are measured using diodes which are not fast enough to resolve 100-picosecond variations in neutron fluxes. To obtain good time resolution, prior art neutron flux detectors using secondary electron emission were small flat plates with relatively small areas for intercepting neutrons. Consequently, they had low detection sensitivity and were found to be unsuitable for application such as measurement of neutrons released in inertial confinement fusion reactions. Even if these prior art detector plates were increased in size, neutrons simultaneously emanating from a point source would have different arrival times. Neutrons which are off-axis will arrive at these targets at a time later than neutrons which are on-axis. Merely increasing the area of these targets thus would result in overlapping of neutron flux information and reduction in the time-resolving ability of a neutron detection system.